This invention relates to a class D amplifier circuit.
A class D amplifier circuit converts an input signal into a pulse width modulation signal of constant amplitude and amplifies power, for example, the class D amplifier circuit is used for power amplification of an audio signal. The class D amplifier circuit operates in binary and thus can drastically lessen the loss of a transistor. Further, the class D amplifier circuit has the advantage that it has higher efficiency than a linear amplifier regardless of whether the amplitude of an input signal is large or small.
The class D amplifier circuit includes, for example, an integration circuit for integrating input signals, a comparison circuit for making a comparison between an output signal of the integration circuit and a predetermined triangular wave signal, and a pulse width amplifier for outputting a pulse width modulation signal pulse-width-modulated based on the comparison circuit. The output signal of the pulse width amplifier is fed back into the input of the integration circuit. The output signal of the pulse width amplifier passes through a low-pass filter made up of a coil, a capacitor, and the like and becomes an analog signal for driving a load of a loudspeaker, etc.
As the class D amplifier circuits, for example, those as disclosed in Patent Document 1 and Non-patent Document 1 are known.
[Patent Document 1] JP-A-2006-217106
[Non-patent Document 1] “Designing and Manufacturing of Class D/Digital amplifier” written and edited by Jun HONDA, CQ Publishing CO., ltd, 2004, p 51-
By the way, such a class D amplifier circuit as described above involves various problems to be solved or considered as described below:
For example, in a half bridge class D amplifier circuit as disclosed in Patent Document 1, it becomes necessary to install a capacitor of a comparatively large capacitance to prevent a DC current from flowing into a load (refer to [FIG. 1] or “AC coupling capacitor” in [Claim 1], etc., of Patent Document 1). Installing the capacitor could be a large hindrance to miniaturization of the whole device, etc. Therefore, preferably, installing the capacitor is circumvented; to realize circumventing of installing the capacitor, for example, a method of supplying positive and negative two power supply potentials to a half bridge circuit is available. However, a new disadvantage in that two power supplies must be provided occurs.
To use a full bridge class D amplifier circuit rather than the half bridge circuit, the problems described above can be suitably solved because a differential signal is given to the full bridge circuit. (For the configuration, refer to p 50, [FIGS. 3-7], etc., of Non-patent Document 1.) That is, a capacitor to prevent a DC current from flowing into a load is not required and positive and negative two power supplies need not be provided. In this case, however, there also occurs a problem in that the D amplifier circuit cannot be applied to a load requiring a ground criterion (for example, stereo headphone, stereo earphone, etc.,) if it is assumed that a unipolar power supply is used, for example.
Further, there is also a problem of a pumping phenomenon in the D amplifier circuit as described above. The pumping phenomenon mentioned here refers to a phenomenon in which the instability of power supply voltage is brought for some cause in a wide sense. More specifically, the instability of power supply voltage is brought because of a response delay of the coil forming a part of the low-pass filter mentioned above (further specifically, for example, a change delay (in the direction) of current relative to voltage change responsive to switching of two switching elements making up the half bridge from one to the other) or the like.